Method for controlling a clearance control valve during a step-climb in cruise phase

ABSTRACT

A method for controlling a clearance control valve of a turbomachine wherein, during a maneuver to increase engine speed in cruise phase, a command to reduce the opening of the clearance control valve is actuated by a Full Authority Digital Engine Control based on a change in the state of a step-climb signal provided by a flight management system in order to increase clearances at the tips of the turbomachine blades and an increase in the opening of the clearance control valve follows its reduction, at the expiry of either of the following two time limits: a first time limit starting at the change in the state of the step-climb signal and determined not to penalize the performance of the engine for too long and a second time limit starting at the end of the maneuver and determined as a function of a thermal time constant of the casing.

TECHNICAL FIELD

The invention relates, in the field of aircraft turbomachines, to thecontrol of clearances at the tips of turbine blades by cooling thecasing and more particularly a method and a system for controlling theclearance control valve during a step-climb in cruise phase.

PRIOR ART

Aircraft turbomachines operating on the combustion principle comprise aturbine to recover the energy from the gases and then convert it intomechanical energy on a drive shaft to drive a fan generating the thrust.The efficiency of the turbine, but also its service life are stronglyconditioned by the seals existing between its different stages. In orderto improve these seals, wipers are generally included at the blade tipscome into contact with an abradable material partially covering thecasing.

However, the relative position of the blade tip (rotor and stator)relative to the abradable material can change during the use of themachine according to various parameters including the variation of thetemperature of the casing which, when cold, contracts, and when hot,expands, causing it to move both axially and radially relative to theblade tip. This is also the case with the variation of the centrifugalforce that leads to a radial displacement of the blade tip and thetemperature variation of the set of the discs and of the shaft itselfthat lead to a mainly axial displacement of the turbine relative to thecasing. To these displacements are added the consumptions of clearanceslinked to the loads and to the dynamics (vibratory dynamics, gyroscopictakeoff force, etc.).

To better control these clearances, it is known to monitor thetemperature of the casing with a regulated supply of fresh air at thecasing by means of a clearance control valve (ACC for Active ClearanceControl) of the turbine. The control of the valve position ensures thatof the air flow rate which in turn monitors the clearances at the bladetip.

The main problem resulting from this solution, based on the cooling ofthe casing, is that the thermal response time of the casing being lowvis-à-vis the clearances linked to the centrifugal force (variation in afew seconds), the clearances are left open as much as possible at alloperating points as well as in transient mode, except in cruise phasewhere these clearances are accurately regulated. However, during thiscruise phase, a request to change the altitude of the aircraft(step-climb), which can be due either to the need to optimize the trip(therefore a planned need) or to a need of avoidance in the vicinity ofan airport requested by the control tower (it is in this case sporadicand corresponds to a much less likely situation on a long-haul type trippassing off the major airports in cruise mode), may occur and force thepilot to push the throttle (action that can also be made automatically)and therefore to cause by this action an overconsumption of abradablematerials reducing the performance that can be expected from the engine.

DISCLOSURE OF THE INVENTION

The present invention therefore proposes to solve this problem of wearat the tips of the blades, during a step-climb in cruise phase.

To do this, there is disclosed a method for controlling a clearancecontrol valve of a turbomachine wherein, during a maneuver to increasethe engine speed in cruise phase, a command to reduce the opening of theclearance control valve is actuated by a Full Authority Digital EngineControl (FADEC) on the basis of a change in the state of a step-climbsignal provided by a flight management system (FMS) in order to increasethe clearances at the tips of the turbomachine blades, the method beingcharacterized in that an increase in the opening of the clearancecontrol valve follows its reduction, at the expiry of either of thefollowing two time limits: a first time limit TBD1 starting at thechange in the state of the step-climb signal (SCs) and determined not topenalize the performance of the engine for too long and a second timelimit TBD2 starting at the end of said maneuver and determined as afunction of a thermal time constant of the casing.

Thus, when the request comes from the FMS, the anticipation of thestep-climb allows preventively increasing the clearances and thusavoiding any contact with the abradable material when the pilot makes ago-around during the cruise phase. This action also allows regainingturbine performance with a high level of confidence.

Preferably, the clearance increase is obtained by a clearance managementsystem comprising the clearance control valve and a device forprojecting fresh air onto a casing of the turbomachine surrounding theblade tips.

Advantageously, the clearance management system comprises a fresh airbleed from a compressor or from a secondary flowpath of the turbomachineor from an air mixture derived from both.

Preferably, the Full Authority Digital Engine Control (FADEC) alsoreceives from the flight management system (FMS) information relating tothe altitude and position of the throttle.

Advantageously, the first time limit TBD1 is much greater than thesecond time limit TBD2 for example, the first time limit TBD1 is on theorder of 15 minutes and the second time limit TBD2 on the order of 30seconds.

The invention also relates to a system for controlling a clearancecontrol valve of a turbomachine, comprising a flight management system(FMS) and a Full Authority Digital Engine Control (FADEC), characterizedin that, during a maneuver to increase the engine speed in cruise phase,the Full Authority Digital Engine Control (FADEC) is able to command areduction in the opening of the clearance control valve on the basis ofa change in the state of a step-climb signal provided by the flightmanagement system (FMS) in order to increase the clearances at the tipsof the turbomachine blades, the system being characterized in that theFull Authority Digital Engine Control is configured to increase theopening of the clearance control valve following its reduction, at theexpiry of either of the following two time limits: a first time limitTBD1 starting at the change in the state of the step-climb signal (SCs)and determined not to penalize the performance of the engine for toolong and a second time limit TBD2 starting at the end of said maneuverand determined as a function of a thermal time constant of the casing.

Preferably, the system for controlling a clearance control valve furtherincludes a distributed digital communication channel connecting theflight management system (FMS) to the Full Authority Digital EngineControl (FADEC).

The invention also relates to a turbomachine including a clearancecontrol valve and such a system for controlling the clearance controlvalve, and to an aircraft including this turbomachine.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages of the present invention willemerge from the detailed description given below, with reference to thefollowing figures without any limiting limitation and wherein:

FIG. 1 partially illustrates several stages of an aircraft turbine atwhich the method for controlling the clearances according to theinvention is applied, and

FIG. 2 is a flowchart illustrating different steps of the method of theinvention.

DESCRIPTION OF THE EMBODIMENTS

FIG. 1 schematically illustrates a partial view of a turbine 10 of anaircraft turbomachine comprising, as illustrated, a rotor 12 providedwith rotor blades 120, 122 and a stator 14 comprising a rectifier 140and a casing 142. Seals are provided between the rotor and the statorwhose clearances, as known, are determined by a clearance managementsystem 16 (LPTACC for Low Pressure Turbine Active Clearance Control orHPTACC for High Pressure Turbine Active Clearance Control) positionedaround the turbine and comprising a clearance control valve 18 (valveACC) and which regulates by distribution (or projection) of fresh airthe temperature of the turbine casing, thereby affecting the clearancesof the seals between rotors and stators of the turbine. It is meant by“fresh air” air taken from a flowpath of the turbomachine, which is at atemperature below the temperature of the casing and which depends on theair bleed location, for example between −60 and +60 degrees for thesecondary flowpath and 200 to 500 degrees for the compressor. It istherefore a relative concept, in comparison the temperature of a turbinecasing is typically regulated between 600 and 900 degrees. Thus, thefresh air can be taken from the compressor or from the secondaryflowpath of the aircraft turbomachine or be constituted by an airmixture derived from these two origins.

The opening of the clearance control valve is regulated by a FullAuthority Digital Engine Control (FADEC) 20 which receives from a flightmanagement system (FMS) 22 the information relating to the altitude andposition of the throttle (thrust lever) actuated by the pilot as well aspossibly other engine parameters such as the engine speed or the Machnumber for example.

The exchanges between the FADEC ensuring the regulation of the engineand the FMS are preferably ensured by a distributed digitalcommunication channel 24 compliant for example with the ARINC(Aeronautical Radio INCorporated) standard.

Given the densification of air traffic, flying in cruise phase is todaypossible only in autopilot mode (also known as auto-throttle). The pilotmust therefore program the FMS 22 at the start of the mission, so as toplan, in an optimal manner, when his flight plan has altitude changes,the step-climb point(s) with respect to the desirable fuel consumption.Thus, during the flight, the FMS 22 can indicate to the pilot themoment(s) of altitude change that then starts with a change in positionof the throttle made by the auto-throttle after authorization of thepilot (or possibly by the pilot himself if he has regained control forwhatever reason). It is also possible that this request for altitudechange comes from the FMS from a ground station for example.

According to the invention, during a step-climb in cruise phase whichwill be engaged by the maneuver to increase the engine speed required bythis altitude change, the clearances at the tips of the turbomachineblades are increased upon receipt of a change in the state of astep-climb signal (SCs) provided by the flight management system (FMS)22, in anticipation of the reduction of the clearances which will resultfrom the action of the auto-throttle or, exceptionally, of the pilot onthe throttle. This increase is made by reducing the opening of theclearance control valve 18 from the Full Authority Digital EngineControl (FADEC) 20, so as to cause a reduction in the projection offresh air on the casing surrounding the blades and therefore itsheating. This heating of the casing will cause its expansion andtherefore increased clearance at the blade tips as desired.

However, it may turn out that the pilot is slow to authorize the actionof the auto-throttle or refuses to give his authorization to operate it,deeming this maneuver inappropriate. This is why an increase in theopening of the clearance control valve follows the previous openingreduction at the expiry of either of the following two time limits: afirst time limit TBD1, preferably on the order of 15 minutes, startingat the change in the state of the step-climb signal and which is acompromise chosen in an optimal manner to take into consideration, onthe one hand, the situation in which the pilot would refuse the maneuver(it is then not necessary to penalize the performance of the engine fortoo long) and, on the other hand, the average reaction time of the pilotto accept this altitude change request; a second time limit TBD2,preferably on the order of 30 seconds, starting at the end of themaneuver and determined as a function of the thermal time constant ofthe casing. It will be noted that the first time limit TBD1 is muchgreater than the second time limit TBD2.

FIG. 2 is a flowchart illustrating the consideration of these two timelimits in the method for controlling the turbine clearances executed inthe FADEC.

In a first step 200, the FADEC periodically recovers from the FMSBoolean information relating to the step-climb signal (True or Falsestate of the step-climb signal). In a next step 202, the FADEC tests thestate of this Boolean information and does not modify its normalinstruction if this state is False, that is to say continues its currentregulation of the clearances while ensuring the monitoring of theopening of the clearance control valve in a step 204. On the other hand,if this state is True, therefore showing a change in the state of thissignal, it is then carried out in a new step 206 at the opening of theclearances. This state changing step executes the start of the timelimit TBD1 during which the pilot is informed of the altitude change andis asked to authorize performing this maneuver automatically. As long asthe long time limit TBD1 has not elapsed, the clearances remain openwhile waiting for the maneuver of the pilot to increase the enginespeed. When the maneuver is finally engaged and then completed, thisfinal step 208 in turn executes the start of the short second time limitTBD2 at the end of which there will be return to the step 204 ofcurrently regulating the clearances. On the other hand, if, at the endof the time limit TBD1, the pilot has not engaged any maneuver, there isautomatic return to step 204 and the current regulation of theclearances is resumed by ensuring again the control of the opening ofthe clearance control valve.

With this method, some gains are obtained from an average fleet ofaircrafts of the long-haul type, although they may not be necessarilyguaranteed for a specific pilot.

1. . A method for controlling a clearance control valve of aturbomachine wherein, during a maneuver to increase the engine speed incruise phase, a command to reduce the opening of the clearance controlvalve is actuated by a Full Authority Digital Engine Control on thebasis of a change in the state of a step-climb signal provided by aflight management system in order to increase the clearances at the tipsof the turbomachine blades, wherein an increase in the opening of theclearance control valve follows its reduction, at the expiry of eitherof the following two time limits: a first time limit starting at thechange in the state of the step-climb signal and determined not topenalize the performance of the engine for too long and a second timelimit starting at the end of said maneuver and determined as a functionof a thermal time constant of the casing.
 2. The method according toclaim 1, wherein the first time limit is much greater than the secondtime limit for example, the first time limit is on the order of 15minutes and the second time limit on the order of 30 seconds.
 3. . Themethod according to claim 1, wherein the clearance increase is obtainedby a clearance management system comprising the clearance control valveand a device for projecting fresh air onto a casing of the turbomachinesurrounding the blade tips.
 4. The method according to claim 3, whereinthe clearance management system comprises a fresh air bleed from acompressor or from a secondary flowpath of the turbomachine or from anair mixture derived from both.
 5. . The method according to claim 1,wherein the Full Authority Digital Engine Control also receives from theflight management system information relating to the altitude andposition of the throttle.
 6. A system for controlling a clearancecontrol valve of a turbomachine, comprising a flight management systemand a Full Authority Digital Engine Control, wherein, during a maneuverto increase the engine speed in cruise phase, the Full Authority DigitalEngine Control is able to command a reduction in the opening of theclearance control valve on the basis of a change in the state of astep-climb signal provided by the flight management system in order toincrease the clearances at the tips of the turbomachine blades, whereinthe Full Authority Digital Engine Control is configured to increase theopening of the clearance control valve following its reduction, at theexpiry of either of the following two time limits: a first time limitstarting at the change in the state of the step-climb signal anddetermined not to penalize the performance of the engine for too longand a second time limit starting at the end of said maneuver anddetermined as a function of a thermal time constant of the casing. 7.The system according to claim 6, wherein the first time limit is muchgreater than the second time limit for example, the first time limit ison the order of 15 minutes and the second time limit on the order of 30seconds.
 8. . The system according to claim 6, further comprising adistributed digital communication channel connecting the flightmanagement system to the Full Authority Digital Engine Control.
 9. Aturbomachine including a clearance control valve and a system forcontrolling the clearance control valve according to claim
 6. 10. Anaircraft including a turbomachine according to claim 9.